1) Field of the Invention
This invention relates to novel salicylic acid resins and metal salts thereof, color-developing agents containing the metal salts as effective components and suited for pressure-sensitive copying paper, as well as color-developing sheets making use of the color-developing agents and suited for use in pressure-sensitive copying paper.
2) Description of the Prior Art
Pressure-sensitive copying paper sheets are also called carbonless copying paper sheets. They produce a color by mechanical or impactive pressure, for example, by writing strokes or typewriter impression, thereby allowing to make a plurality of copies at the same time. Among such pressure-sensitive copying paper sheets, there are those called "transfer type copying paper sheets", those called "self-contained copying paper sheets", etc. Their color-producing mechanisms are each based on a color-producing reaction between an electron-donating colorless dyestuff precursor and an electron-attracting color-developing agent. Taking a pressure-sensitive copying paper sheet of the transfer type by way of example, it will be described with reference to FIG. 1 which is a schematic cross-sectional view showing the structure of the exemplary pressure-sensitive copying paper sheet.
The back sides of a CB-sheet 1 and CF/CB-sheet 2 are coated with microcapsules 4 which have diameters of several micrometers to somewhat greater than 10 micrometers and have been obtained by dissolving a colorless pressure-sensitive dyestuff precursor in a non-volatile oil and then encapsulating the resultant solution with high-molecular films such as gelatin films. On the other hand, the front sides of the CF/CB-sheets 2 and a CF-sheet 3 are coated with a coating formulation containing a color-developing agent 5 which has such properties that upon contact with the pressure-sensitive dyestuff precursor, the color-developing agent 5 undergoes a reaction with the dyestuff precursor, thereby causing the dyestuff precursor to produce its color. In order to make copies, they are stacked in the order of the CB-sheet, (CF/CB-sheet), (CF/CB-sheet) and CF-sheet with the sides coated with the dyestuff precursor maintained in contiguous relation with the sides coated with the color-developing agent. The CF/CB-sheets are optional. When a pressure is applied locally by a ballpoint pen 6 or a typewriter, the capsules 4 are ruptured there. As a result, the solution containing the pressure-sensitive dyestuff precursor is transferred to the color-developing agent 5, so that one or more copied records are obtained.
As electron-attracting color-developing agents, there have been proposed (1) inorganic solid acids such as acid clay and attapulgite, as disclosed in U.S. Pat. No. 4,173,684; (2) substituted phenols and diphenols, as disclosed in Japanese Patent Publication No. 9309/1965; (3) p-substituted phenol-formaldehyde polymers, as disclosed in Japanese Patent Publication No. 20144/1967; (4) metal salts of aromatic carboxylic acids, as disclosed in Japanese Patent Publication Nos. 10856/1974 and 1327/1977; etc. Some of them have already been employed actually.
As performance conditions which a color-developing sheet is supposed to satisfy, may be mentioned little yellowing during storage and upon exposure to radiant rays such as sunlight and good fastness of produced color marks so that they do not disappear or fade easily by radiant rays, water or a plasticizer, to say nothing of excellent color-developing ability not only right after its fabrication but also after its storage over a long period of time.
Color-developing agents, which have been proposed to date, and sheets coated with such conventional color-developing agents have both advantages and disadvantages in performance. For example, inorganic solid acids are inexpensive but adsorb gas and moisture in the air during storage. They hence result in yellowing of paper surfaces and reduced color-producing performance. Substituted phenols have insufficient color-producing ability and produced color marks have low color densities. Para-phenylphenol-novolak resins which are usually employed as p-substituted phenolformaldehyde polymers have excellent color-producing ability, but their coated paper sheets undergo yellowing and produced color marks are faded significantly upon exposure to sunlight or during storage (especially, by nitrogen oxides in the air). In addition, metal salts of aromatic carboxylic acids are good in yellowing resistance but their color-producing ability at low temperatures, resistance to water or plasticizers and light fastness cannot still be considered sufficient.